Electronic article surveillance ("EAS") systems detect the presence of small electronic devices placed on or in an article or carried by a person of interest, and are often used in retail or library environments to deter theft or other unauthorized removal of articles. These devices, which are commonly known as tags or markers, have in the past contained only information regarding the presence of an item. This information could be obtained by electronically interrogating the tag, either intermittently or continuously. At least four distinct types of EAS systems have evolved over the years, based on how this interrogation was carried out: magnetic, magnetomechanical, radio frequency (RF), and microwave. Of these four, magnetic systems have provided the highest level of security in most applications. Magnetic tags are easily hidden in or on an object, difficult to detect (because they are less susceptible to shielding, bending, and pressure), and easy to deactivate and reactivate, thereby providing a high degree of security and some information regarding the status of the tagged article.
Many users of EAS systems desire to know more than just whether a tagged object is present. They also want to know which tagged object is present, for example. Detailed information regarding the characteristics of objects, such as their date of manufacture, inventory status, and owner have generally been communicated to automated handling and control systems through an optical bar code. While inexpensive and effective, the optical bar code system has certain limitations. Bar codes must be visible, which limits the locations in which they may be placed, and bar codes can easily be obscured, either accidentally or intentionally. The range at which a detector can sense the bar code is also comparatively small. The bar code may also have to be appropriately positioned for detection. Also, because bar codes are often exposed to permit detection, the barcode is susceptible to damage that can result in detection failures. Lastly, multiple items must be processed one at a time. These constraints of bar code systems make them undesirable or inefficient for some applications, such as marking library media.
More recently, electronic identification (also known as radio frequency identification or RFID) techniques have been developed to address the limitations of optical barcodes. RFID systems have succeeded in providing object identification and tracking, but are deficient in providing object security because most RFID systems operate in frequency ranges (.about.1 MHz and above) in which the tag is easily defeated. The security deficiency associated with radio frequency tags arises because they can be "shielded" by, for example, covering the tag with a hand or aluminum foil, or even placing the tag in a book. Even battery-powered radio frequency tags may be blocked, although their range is superior and blocking would be more difficult. Thus, objects tagged with an RFID tag may escape detection, either inadvertently or intentionally. This greatly reduces their effectiveness as security devices. RFID markers are also related to "smart cards." Both contact and contactless smart cards have appeared in commercial applications. Smart cards tend to be associated with a specific person rather than with a tagged object. Issues related to the security and tracking of the smart card (or of the person carrying it) are similar to those discussed above for RFID markers.
The security issues associated with RFID markers are similar to those familiar to anyone skilled in the art of radio frequency- and microwave-based EAS tags. Substantial effort has been expended in attempts to remedy the deficiencies of radio frequency- and microwave-based EAS tags. However, none has substantially improved their performance as security tags. U.S. Pat. No. 5,517,195 (Narlow et al.), entitled "Dual Frequency EAS Tag with Deactivation Coil," describes a dual frequency microwave EAS tag that includes an antenna circuit having a diode, and a deactivation circuit. The deactivation circuit responds to a low energy alternating magnetic field by inducing a voltage in the diode of the antenna circuit so as to disable the diode and the antenna, thereby deactivating the tag. Although useful for some applications, the capacitor-based tag disclosed in Narlow et al. may leak electrical charge over time, which could cause the tag to become activated unintentionally.
Radio frequency EAS tags of the type disclosed in U.S. Pat. No. 4,745,401 (Montean et al.) include a magnetic element. The magnetic element alters the tuning of the tag when it has been suitably magnetized by an accessory device, and thereby blocks the radio frequency response of the tag. Although these tags have a certain utility, they still do not address the issues of enhanced security and identification.
Radio frequency identification technology has been developed by a number of companies, including Motorola/Indala (see U.S. Pat. Nos. 5,378,880 and 5,565,846), Texas Instruments (see U.S. Pat. Nos. 5,347,280 and 5,541,604), Mikron/Philips Semiconductors, Single Chip Systems (see U.S. Pat. Nos. 4,442,507; 4,796,074; 5,095,362; 5,296,722; and 5,407,851), CSIR (see European document numbers 0 494 114 A2; 0 585 132 A1; 0 598 624 A1; and 0 615 285 A2), IBM (see U.S. Pat. Nos. 5,528,222; 5,550,547; 5,521,601; and 5,682,143), and Sensormatic Electronics (see U.S. Pat. No. 5,625,341). These tags all attempt to provide remote identification without the need for a battery. They operate at frequencies ranging from 125 KHz to 2.45 GHz. The lower frequency tags (.about.125 KHz) are moderately resistant to shielding, but have only limited radio frequency functionality due to bandwidth constraints. In particular, systems based on these markers generally operate reliably only when a single tag is in the interrogation zone at a time. They also tend to be relatively bulky and expensive to manufacture. At higher frequencies, (typically 13.56 MHz, 915 MHz, and 2.45 GHz), the added bandwidth available has permitted the development of systems which can reliably process multiple tags in the interrogation zone in a short period of time. This is highly desirable for many product applications. In addition, some of the tag designs hold the promise of being relatively inexpensive to manufacture and therefore more attractive to a customer. However, these higher frequency devices share to varying degrees the susceptibility to shielding discussed earlier. Thus, they cannot provide the high level of security demanded in certain applications, such as a library.
From the foregoing discussion, it should be clear that there are a number of applications for RFID tags in various environments in which the identity of the tagged item is important. For example, PCT Publication WO 99/05660, published Feb. 4, 1999 and assigned to Checkpoint Systems, Inc., describes an inventory system using articles with RFID tags. The preferred embodiment described therein contemplates the use of RFID tags in library materials, which may then be checked out automatically by interrogating the RFID tag to determine the identity of the material. However, a number of important or desirable library or other inventory functions remain that are not described or suggested in the '660 publication.